预期类型 `a' 带有 `Double' 错误：Haskell

Question

I'm writing a program that returns a list of the k nearest neighbours, in distance order, according to metric d to point p in the list ps of points. All should be returned as proper neighbours if in the list ps are fewer than k elements. Also, two equally close points to p should be returned based on their initial order in the list ps

import Data.List

type Point a = (a,a)
type Metric a = (Point a) -> (Point a) -> Double

nearPoints ::  Int -> Metric a -> Point a -> [Point a] -> [Point a]
nearPoints k d p [] = []
nearPoints k d p ps = take k (sortOn (pointsDistCalc p) ps)

pointsDistCalc :: Metric Double
pointsDistCalc (t1,t1) (t2,t2) = sqrt(((t1-t2)^2)+((t1-t2)^2))

This is the error message

    * Couldn't match type `a' with `Double'
      `a' is a rigid type variable bound by
        the type signature for:
          nearPoints :: forall a.
                        Int -> Metric a -> Point a -> [Point a] -> [Point a]
      Expected type: Point Double
        Actual type: Point a
    * In the first argument of `pointsDistCalc', namely `p'
      In the first argument of `sortOn', namely `(pointsDistCalc p)'
      In the second argument of `take', namely
        `(sortOn (pointsDistCalc p) ps)'
    * Relevant bindings include
        ps :: [Point a]
        p :: Point a
        d :: Metric a
        nearPoints :: Int -> Metric a -> Point a -> [Point a] -> [Point a]
11 | nearPoints k d p ps = take k (sortOn (pointsDistCalc p) ps)

Please can someone help me with this

Answer 1

You defined pointsDistCalc as a Metric for Point Double s, hence that means that nearPoints which sort by that distance, should be Point Double s as well.

That being said, the signature suggests that a metric is given, and thus that one does not per se uses the PointsDistCalc . You thus can implement this with:

nearPoints ::  Int -> Metric a -> Point a -> [Point a] -> [Point a]
nearPoints _ _ _ [] = []
nearPoints k  p0 ps = take k (sortOn ( p0) ps)

There is no need to split the definition in two cases here: sortOn will return an empty list if it is given an empty list:

nearPoints ::  Int -> Metric a -> Point a -> [Point a] -> [Point a]
nearPoints k  p0 ps = take k (sortOn ( p0) ps)